This invention relates to the control of air and pulverized coal supplied to the burners of a coal-fired boiler and, specifically, to automatic coal dampers and related control systems.
Coal-fired boilers typically have multiple arrangements of coal pulverizing mills, each mill supplying coal through multiple pipes to multiple burners within the boiler. Each parallel coal supply path typically originates at a respective pulverizer mill and terminates at the individual burner mounted in the boiler. Each coal pipe has its own characteristic mechanical system performance/resistance properties for the two phase flow of air flow and coal flow, and this varies for each parallel coal pipe path at any given time and boiler load, based upon a number of system factors relating to both equipment and process variables. For example, equipment such as a forced draft fan, air heater, primary air fan, coal feeder, coal pulverizer, coal classifier, riffle box/fixed orifice, piping system, elevation, air flow and coal flow monitor, coal damper, burner isolation valve, burner, boiler, and process parameters such as elevation, air temperature, air pressure, air flow, coal flow, coal density, coal moisture, coal composition and coal particle size all impact the performance/resistance characteristics of the system. In other words, as the boiler load changes and as the individual mechanical factors vary for each coal pipe, the air/coal system performance and resistance changes for the total coal piping system and each individual coal pipe within that system.
It is known that the balance of coal flow to the burners in a coal-fired power plant can have significant impacts on combustion efficiency, residual carbon in fly ash, and NOx emissions. Even a small burner-to-burner imbalance can significantly impact boiler performance. Coal flow balancing of multiple burner boilers is a difficult problem for engineers and operators to solve. It is typically performed as an iterative series of manual coal flow measurements and adjustments of flow restrictive devices in the coal piping. With the introduction of manual coal dampers, coal flow has been balanced by adjusting each manual damper in each of the pulverized coal pipes that supplies the burners from a single mill. The coal flow rates in each pipe are measured manually by sampling with a coal probe traversing across the coal pipe area. While this approach had the potential to achieve approximate balance, changes in fuel consumption, operating conditions and wear on the orifice plates resulted in uncontrollable coal flow balance variations.
Real time coal flow monitoring systems are now available and are described in the patent literature. See, for example, U.S. Pat. Nos. 6,109,097; 6,289,266; and 5,048,761. Variable orifices for coal pipes are disclosed in U.S. Pat. Nos. 5,975,141; 5,685,240; 6,009,899 and 6,234,090. Presently, however, there is no known coal damper arrangement which links real time air flow and coal flow signals to changes in the primary air/coal flow system resistance and the functional process of changing the coal damper position to an optimum operating set point.
The automatic coal damper in accordance with this invention provides a more practical method of achieving coal flow balance conditions for a boiler. The automatic coal damper also allows coal flow adjustments based upon varied boiler load, and may apply neural network technology. It also operates for particular coal flow settings which may optimize the boiler combustion and air emission levels.
Specifically, this operating scheme is designed to give operators the ability to balance coal flow from a mill so as to distribute the coal equally among the burners of the coal-fired boiler. A well-balanced coal flow system should improve boiler efficiency, minimize NOx and CO formation, while achieving maximum or desired Kilowatt production. The associated performance data will interface with the power plant control and/or data acquisition system to permit real time online adjustment of the coal dampers to thereby balance the coal flow among the various parallel paths to the burners.
In the exemplary embodiment, the individual coal damper position is changed automatically based on real time coal flow signals to provide the optimum operating set point for each coal damper. In other words, the automatic coal damper balances the system resistance, allowing the coal flow of each burner to be maintained at an optimum operating set point. Ideally, the optimum operating set point is usually, but not necessarily, an equal air flow for each individual burner of the coal-fired boiler.
There are two new orifice plate designs disclosed herein for the automatic coal damper: dual action orifice plates and quad action orifice plates (and variations of each) adjusted by, for example, an electric motor linear actuator. However, the invention here is also applicable to known single action orifice plates, where one slidable plate is adjusted manually and the other plate is adjusted by the actuator. For the dual action orifice plates design in accordance with this invention, both plates swing simultaneously, at the same angular position to any point between a fully closed position and a fully open position. The size of the orifice opening at the fully closed position is set at a certain percentage of the full open position, but the orifice opening size and shape (for example, diamond, circle, oval or other) may be customized for the requirements of a particular boiler installation. A self-cleaning feature may be included for the dual action orifice plates design, in that the plates may swing 180xc2x0 from a full up and open position beyond the full closed position, to a full down position such that any coal build up on the normally top side of the plates may be removed by gravity or primary air pressure.
The automatic coal damper normally installed in vertical sections of coal piping, usually near the exit of the coal pulverizer.
The mechanical drive mechanism for adjusting the dual action orifice plates includes an electric motor linear actuator that pushes a rod attached to a cam that has dual slots, one for each of the two driven orifice plates. Each slot captures a roller which is linked at the end of a fulcrum arm on a cam rotor. The cam rotors rotate from zero to ninety degrees and the orifice plates similarly rotate from zero to ninety degrees. Each orifice plate is welded (or otherwise fixed) to a shaft and the two shaft ends are supported by two bearings which are mounted externally on opposite sides of the damper body. The shaft ends are fixed to the opposite ends of the fulcrum arm so that the cam rotors and orifice plates rotate simultaneously through the same angle.
For the quad action orifice plates design, the same mechanical drive mechanism with two swing orifice plates as described above is used, but additionally, there are two idler orifice plates which are connected to the two adjacent driven orifice plates by two connecting, triangular-shaped hinged plates. The advantage of the quad action orifice plates is that the orifice remains symmetrically centered within the coal pipe.
Ceramic sleeve hinges are used to connect the driven orifice plates to the idler plates at four locations, allowing the idler plates to rotate freely about the hinges in the abrasive and hot environment typical of coal piping applications. The ceramic sleeve arrangement also prevents coal from binding the rotation action of the pivot shafts. The two triangular shaped orifice plates at each interface of a driven plate and an adjacent idler plate fold together in the fully closed orifice position and fold completely apart in the fully open orifice position. The size and orientation of the orifice plates may be customized for the requirements of a particular boiler installation.
In an alternative and presently preferred damper body design, the damper is a dual plate type, where the body housing is round rather than rectangular or square. This design thus maintains the same cross-sectional shape for the flow path through the damper as in the coal piping on either side of the damper.
In the control process, a known coal flow monitor measuring system takes velocity and coal flow measurements from each pipe, and a suitable programmable logic controller, integrated with the measuring system, makes small percentage adjustments to each damper, when necessary, to balance out the coal flow in each pipe.
Accordingly, in one aspect, the invention relates to apparatus for controlling primary air flow and pulverized coal flow to a plurality of burners in a coal-fired boiler comprising a plurality of coal dampers arranged to supply a mixture of air and pulverized coal to respective burners in the coal-fired boiler, each damper having a damper body and at least two orifice plates pivotally secured therein, the orifice plates movable between open and closed positions; a real time coal flow monitoring device operatively associated with each damper that is adapted to generate analog signals representing real time coal flow through its respective damper; and a programmable logic controller adapted to receive the analog signals and to adjust the orifice plates to balance the flow of air and pulverized coal to each of the plurality of burners.
In another aspect, the invention relates to an automatic coal damper for use in controlling flow of primary air and pulverized coal to a burner of a coal-fired boiler comprising a damper body having pipe sections on either end thereof, the damper body having at least two adjustable orifice plates secured to respective pivot shafts mounted in the damper body, and a linear actuator mounted on the damper body and operatively connected to the pivot shafts for moving the orifice plates between open and closed positions.
In still another aspect, the invention relates to a coal-fired boiler plant comprising plural parallel piping paths from at least one pulverizer mill that supply air and pulverized coal to a corresponding plurality of burners of a coal-fired boiler; a plurality of mechanical dampers located, respectively, in the plural parallel piping paths, each damper having at least a pair of pivotally adjustable orifice plates; and means for adjusting the orifice plates in real time to balance the flow of air and pulverized coal for each of the plural parallel piping paths.
The invention will now be discussed in detail in connection with the drawing figures briefly described below.